The present invention relates to a method and an apparatus for converting a scan value of an input scanning sequence occurring with an input scanning frequency into scan values of an output scanning sequence occurring with an output scanning frequency.
One such method and apparatus is already known from European Patent Application No. 0052 847. In this method and apparatus, the input scanning sequence is converted into the output scanning sequence by multiple digital filtering of the input scan values, in a multiple digital filter. The input scanning sequence is converted in several steps into the output scanning sequence. This process is controlled by the momentary time difference between the input and the output scanning times. This difference is used for selecting a momentary filter coefficient set. An important part of this apparatus is therefore provided for determining this time difference.
Hitherto known apparatuses for determining the time difference between input and output scanning times are unable to follow rapidly changing input and output scanning frequencies. This is on the one hand because the intermediate store of a multistage scanning converter is small and on the other hand the processing of time-varying scanning frequencies leads to time differences between the momentarily effectively present scanning values and the calculated time differences between the particular input and output scan values. Thus, in the case of rapidly changing scanning frequencies, the intermediate store can no longer intermediately store sufficient scan values, which means that the apparatus can no longer fulfill its function. A further disadvantage of the multistage scan rate converter is that in the case of continuously changing input and output scanning frequencies, in which both the output and input scanning frequency can assume higher and lower values, the operating sequence of the individual stages of the multistage digital filter must be interchanged, which cannot take place without interrupting operation.